1. Field of Invention
The invention relates generally to the field of cameras and of illumination devices, and more particularly to a camera having an integrated illumination device adapted for enhanced illumination.
2. Description of Related Art
Cameras, particularly video cameras used in surveillance applications, may be provided with illuminators to provide adequate light for the camera to produce a discernable image. In certain applications, the camera can detect infrared light not visible to the naked eye. Accordingly, infrared light may be provided by an illuminator, allowing the camera to image the scene across both the infrared and visible spectra without producing visible light. Generally, the illuminators are positioned to direct light along an illumination axis parallel to the central axis of the camera.
There are, however, certain problems with such arrangements. First, light intensity falls off as the square of distance. This presents a problem when the camera is positioned to image a field of view that includes various depths. As the intensity of the illumination is increased to provide adequate light for the distant areas within the field of view, the high intensity light in the areas closer to the camera overloads the image, whereby any light reflected from the closer areas is at maximum intensity, eliminating any contrast in that portion of the image and producing a phenomenon called “washout”.
Second, light intensity also falls off as the position deviates from the illumination axis. Referring to FIG. 4, illustrated is a polar plot of relative radiant intensity v. angular displacement for a typical LED illuminator. The illustrated illuminator produces a cone of light, defined by the cone angle between the illumination axis and a certain light intensity threshold. For example, the illuminator graphed in FIG. 4 demonstrates a cone of approximately 20° to either side of a central illumination axis in the region nearest the source. This illuminator could be described by the cone angle it produces, i.e. 20°.
This characteristic of certain illuminators contributes to the washout phenomenon, particularly by creating a “hotspot”, or a central area of the image where in illumination overloads the camera's ability to produce an image. In the alternative, if the illumination intensity is reduced to control the hotspot and washout, the light is insufficient to create a discernable image in the areas distant from the camera.